1. Field of the Invention
The invention relates to devices for monitoring the emission wavelength of a laser.
2. Discussion of the Background Art
Commercial WDM (Wavelength Division Multiplex) transmission systems, such as xe2x80x9cdensexe2x80x9d WDM (DWDM) systems provide high transmission capacity by using a channel spacing of 100-50 GHz. Real time wavelength monitoring and control is thus necessary in order to ensure the wavelength stability required for the optical source.
A number of arrangements adapted for that purpose are disclosed e.g. in U.S. Pat. No. 5,781,572, after which the preamble of claim 1 was patterned.
In addition to a laser emitting the radiation whose wavelength is to be controlled, such arrangements include at least one wavelength selective optical component, a device (such as a Peltier element) for controlling a temperature of the laser diode as well as a drive circuit for driving the Peltier element. The radiation from the laser (which is derived from the main radiation beam of the laser) is split over two paths. Wavelength selective optical components are arranged in at least one of the two optical paths. These components are optical filters having spectral characteristics whose transmittance continuously changes as a function of wavelength. Radiation passing through the optical filters is detected by means of photodetectors. The signal thus generated are used to drive the Peltier element.
A somehow similar arrangement is disclosed in EP-A-0 284 908 and the corresponding U.S. Pat. No. 4,815,081. There, an arrangement for controlling or regulating the emission wavelength and emitted optical power of a semiconductor laser is disclosed adapted for integrated optics implementation. In that arrangement, the laser, the filter device and first and second photodetectors (in the form of photodiodes) are integrated on a common substrate. The integrated filter device comprises one or more Bragg gratings, an optical directional coupler or an interference filter arranged in series in the directions of propagation of the conductor power. The optical powers which are conducted to the integrated filter device and to the first and second photodiodes are guided in each instance in an optical waveguide which is integrated on the substrate in the form of a strip waveguide.
In U.S. Pat. No. 6,094,446 an arrangement is disclosed wherein the light emitted by a laser diode is directed towards an interference optical filter. The light passing through the filter and the light reflected therefrom are detected by respective photodiodes.
Similarly, in the arrangement disclosed in U.S. Pat. No. 6,134,253, an optical filter is provided for receiving the laser radiation and for transmitting and reflecting first and second filtered beams respectively. The beams are filtered according to respective first and second spectral filter functions that cross at least one crossing wavelength. Varying the angle of incidence of the laser radiation upon the optical filter varies the spectral function thereof thus selecting or varying the operating wavelength of the laser.
Another arrangement of interest for the invention is disclosed in EP-A-0 818 859 (i.e. U.S. Pat. No. 5,825,792).
In order to manufacture a compact wavelength stabilised laser source a number of issues must be taken into account.
For instance, the wavelength selective components must be operated under optimum or near-to-optimum conditions and, preferably, should be temperature controlled in order to avoid drift of their wavelength locking point with temperature.
In order to collect more power, the radiation to be detected must be generally collimated to give rise to a low-divergence beam by using a lens, possibly requiring active alignment as disclosed by K. Anderson in IEEE Electronic Component and Technology Conference, 1999, p. 197-200.
The requirements outlined in the foregoing may however result in arrangements which are overly complex and expensive to manufacture.
The object of the present invention is to provide an improved system overcoming the drawbacks of the prior art solutions considered in the foregoing, which is compact and adapted to be co-packaged within the same package with a laser emission source, by overcoming coupling, space and power dissipation problems, while also permitting feedback to the wavelength control to take place effectively over the full temperature operating range of the laser source.
According to the present invention that object is achieved by means of a device having the further features called for in the claims which follow.
Essentially, the arrangement of the invention consists of a wavelength monitoring system based on a wavelength selective element (typically an interference filter) for detecting an error signal and adapted for use in a control system using temperature feedback. The optical filter is deposited on a Si slice, placed on a Silicon Optical Bench (SiOB). The Si slice or slab preferably has a wedge shape and is arranged in a vertical position with the beam from the laser source impinging thereon under a high incidence angle. The slice position/orientation is determined in such a way to split the optical signal over two paths, which are collected by two photodetectors. One of these paths passes through the Si slice and the optical filter.
The high refractive index of Si (and the shape of the slice) collimate this beam thus optimising power transfer. Therefore, the incidence angle is determined in order to maximise the transmission at the air-Si and Si-air interfaces, working at the Brewster angle, whereby the incidence angle on the interference filter is almost constant and very close to 0xc2x0 (normal incidence).
The SiOB platform is fully compatible with modern production processes, while allowing an efficient temperature control of the optical filter and simplifying mounting of the optical elements. Specifically, the arrangement of the invention is thoroughly compatible with the passive alignment process of the optical mount, thus implying lower costs.